From what I know about the family, Grace wasn't the sister of Charles, Jean etc, she was in fact the aunt of them all so it puts Jean after Charles and before the first James to be born in 1911.
However, she wasn't adopted as far as we are aware as the details from her death certificate give month and year of birth as June 1909.
Another thought I had was that Grace and Jean were in fact the same person, but looking at Grace's marriage and death records seem to disprove it.